1. Field of the Invention
The present invention relates to an apparatus and a process for mounting conductor balls, on terminal pads of semiconductor devices or other electronic devices collectively formed in a single semiconductor wafer or other substrate, stepwise for divisions of the substrate that include different numbers of the devices. The present invention also relates to an adsorbing unit advantageously used in the apparatus and a process and a mask plate suitably used in the adsorbing unit.
2. Description of the Related Art
FIG. 1 shows a conventional semiconductor device 100 of a BGA (ball grid array) type having ball form terminals or solder balls 110 bonded to terminal pads 108 to form external connection terminals on one side of a circuit board 102. The terminal pads 108 form one end of conductor wiring patterns 106 having the other ends wire-bonded to a semiconductor chip 104 on the other side of the circuit board 102.
To mount the solder balls 110 on the terminal pads 108, U.S. Pat. No. 5,284,287 discloses an adsorbing unit for adsorbing the solder balls 110 as shown in FIG. 2.
The adsorbing unit 100 includes an adsorbing plate 114 having adsorbing holes 112 penetrating therethrough at positions corresponding to the terminal pads 108 of the circuit board 102 and having a diameter smaller than that of the solder balls 110, in which the adsorbing plate 114 is connected to a vacuum pump system for evacuating the adsorbing holes 112 to adsorb the solder balls 110 at an open end thereof.
As shown in FIG. 3, the adsorbing holes 112 are evacuated to adsorb the solder balls 110 contained in a vessel 116 with one adsorbing hole 112 adsorbing one solder ball 110 and the adsorbing plate 114 is then moved to bring the adsorbed solder balls 110 into close contact to the corresponding terminal pads 108 where the adsorbing holes 112 are freed from evacuation to release the solder balls 110 onto the terminal pads 108, thereby mounting the solder balls 110 on the terminal pads 108.
As shown in FIG. 4, a chip size package (CSP) 200 has substantially the same size as the semiconductor chip 202 packaged therein and having on one side a conductor pattern 206 with one end electrically connected to an electrode terminal 204 of the chip and the other end forming a terminal pad on which a solder ball 208 is mounted to provide a ball-form external connection terminal. The conductor pattern 206 is formed on one side of a resin film 210. A resilient resin layer 212 is interposed between the conductor pattern 206 and the surface of the semiconductor chip 202, on which surface the electrode terminal 204 is formed, to absorb thermal stress generated by the different thermal expansion coefficients of the semiconductor chip 202 and a mother board on which the CSP 200 is mounted.
The above-mentioned end of the conductor pattern 206 connected to the electrode terminal 204 protrudes from an edge of the resin film 210 and is sealed or packaged with the electrode terminal 204 by a packaging resin 214.
The CSP 200 shown in FIG. 4 is smaller than the BGA type semiconductor device 100 shown in FIG. 1 and is manufactured, as shown in FIG. 5, by forming many CSPs 200 on many semiconductor elements formed in the wafer 300 and then dicing or cutting the wafer 300 into respective CSPs 200. FIG. 5(a) shows the whole area of the wafer 300 and FIG. 5(b) is an enlarged partial view showing the circled part P of the wafer 300.
Before dicing the wafer 300, the solder balls 208 are mounted on terminal pads of the above-formed semiconductor devices or CSPs 200. The solder balls 208 are mounted quickly by using the adsorbing unit shown in FIGS. 2 and 3.
The wafer 300 is imaginarily divided into plural divisions 302a to 302l having the same shape and size as that of the adsorbing plate 114 of the adsorbing unit and the mounting of the solder balls 208 is carried out division by division, because the adsorbing plate 114 has a limited actually producible size while ensuring the necessary flatness and other accuracies.
The wafer 300 includes twelve divisions 302a to 302l, in which four divisions 302c, 302d, 302i and 302j have an area occupied by a group of the semiconductor devices 200 (corresponding semiconductor elements 202 are shown in FIG. 5) that completely fills each of these four divisions, so that the device-occupied areas of these four divisions have the same shape and size, i.e., have a single pattern. In contrast, eight divisions 302a, 302b, 302e, 302f, 302g, 302h, 302k and 302l have an area occupied by the semiconductor devices 200 that incompletely fills each of these eight divisions, so that the device-occupied areas of these eight divisions are different in shape and size between divisions, i.e., have eight patterns.
To mount solder balls 208 on terminal pads of semiconductor devices 200 arranged in nine different patterns, it is necessary to prepare adsorbing plates 114 having adsorbing holes 112 arranged in nine different patterns.
The adsorbing plate 114 is very expensive to produce and the cost of producing semiconductor devices is significantly raised by preparing various types of adsorbing plates.
The adsorbing unit shown in FIGS. 2 and 3 also has a drawback in that the adsorbing plate 114 is fixed to the unit body and exchange of the adsorbing plate 114 is not possible, so that it is extremely difficult, or actually impossible, to quickly mount solder balls 208 on the desired terminal pads with quickly exchanging adsorbing plates 114.